Combustion catalyst



COIWBUSTION CATALYST Wayne A. Proell, Chicago, EL, and William G. Stanley, Hammond, llnd., assignors to Standard Oil Company, Chicago, 11]., a corporation of Indiana No Drawing. Filed May 15, 1952, Ser. No. 288,065

4 Claims. (Cl. 52-14) This is a continuation-in-part of our copending application, Serial No. 273,564, filed February 26, 1952, and relates to new and improved explosive compositions and also to compositions for the generation of gas at high pressure. Particularly, the invention relates to explosive compositions wherein ammonium nitrate is the principal or sole gas-producing agent. Still more particularly, the invention relates to an explosive comprising ammonium nitrate, an oxidizable material and a combustion catalyst.

Ammonium nitrate is widely used as a component of high explosives. Although ammonium nitrate is classified as a high explosive, it is extremely insensitive and cannot readily be detonated by the local application of heat or by a blasting cap; and when ignited, ammonium nitrate does not sustain propagation consistently. Normally ammonium nitrate is mixed with an oxididable material, such as, sulfur, carbon, cellulosic materials, hydrocarbons, etc., in order to utilize the excess oxygen available in the ammonium nitrate. However, these mixtures of ammonium nitrate and oxidizable materials are also either very insensitive or slow burning.

One of the well known methods of overcoming this lack of sensitivity is to use a sensitive high explosive to prime the detonation of the ammonium nitrate explosive. Examples of suitable primers are tetryl, TNT, nitrostarch, nitrocellulose, nitroglycerine, etc. An explosive that is detonable by the action of a blasting cap can be obtained by mixing the ammonium nitrate with the sensitive materials, such as, nitrostarch and nitrocellulose. The extreme sensitivity of these explosives makes them undesirable for ordinary blasting use.

Another method of obtaining a readily ignitable ammonium nitrate explosive is to admix therewith an organic sensitizer, such as, nitrogen compounds and certain carbohydrates. In general, satisfactory explosives are obtained only when the sensitizer is very intimately dispersed throughout the mass. Generally this dispersion is obtainable only by the use of complicated and expensive procedures.

The most commonly used method for improving the sensitivity of ammonium nitrate explosives is to add a combustion catalyst. The commercially used combustion catalysts are all based on the element chromium. The more common chromium combustion catalysts are ammonium or alkali metal chromates or polychromates; chromic oxide, chromic nitrate and copper chromite. The preferred material is ammonium dichromate. While the chromium compounds are the best known combustion catalysts, they have the disadvantages of being expensive and of, frequently, being in very short supply. The relative toxicity of the chromates makes them hazardous unless handled with considerable care. A particular disad vantage to the use of chromium combustion catalysts is their strong oxidizing properties, such that they tend to react with the binder upon long storage to give chromium salts which are relatively ineitective catalytically.

An object of this invention is the preparation of a new and improved inorganic nitrate explosive, in particular,

an ammonium nitrate explosive. Another object is the preparation of an explosive comprising ammonium nitrate and a combustion catalyst. Still another object is to provide a combustion catalyst for ammonium nitrate explosives which is cheap and in plentiful supply. A particular object of this invention is an explosive mixture comprising ammonium nitrate, an oxidizable material and a particular burning catalyst, which mixture can be formulated by a hot melt process. A further object of this invention is a gas generating composition for the generation of gases at pressures on the order of 1,000 p.s.i., which composition comprises ammonium nitrate, an oxidizable material and an insoluble Prussian blue combustion catalyst.

Our copending application, Serial Number 273,564, filed February 26, 1952, discloses an ammonium nitrate explosive containing a combustion catalyst selected from the group consisting of iron-iron cyanide complexes, alkali metal-iron-iron cyanide complexes and ammonium-ironiron-cyanide complexes. It was stated therein that the preferred combustion catalyst was soluble Prussian blue. The soluble Prussian blue containing explosive mixture is readily ignited at atmospheric pressure and has the best burning rate, inches per second, at atmospheric pressure. We have discovered that the burning rate of an ammonium nitrate-oxidizable material-combustion catalyst mixture at about 1,000 p.s.i. pressure is higher for the mixtures containing a catalyst selected from the group consisting of ferric ferrocyanide and insoluble Prussian blue than is the burning rate for an explosive mixture of the same composition, wherein the catalyst is one of the various soluble Prussian blues. This increase in burning rate at elevated pressures, such as would be used in military rockets or gas generating apparatus for the insoluble Prussian glue catalyzed mixtures is unexpected and surprising. Thus for uses where the gas generating composition will operate at pressures on the order of 1,000 p.s.i., and it is desirable to have a larger volume of gas produced per unit time from a combustion chamber of fixed 'size, it is preferred to use an ammonium nitrate mixture containing an effective amount of a catalyst selected from the group consisting of ferric ferocyanide and insoluble Prussian blue. Insoluble Prussian blue has other advantages over soluble Prussian blue. The insoluble Prussian blue is cheaper and, more important, it is more uniform in catalytic activity.

, The insoluble Prussian blue containing ammonium nitrate mixture is extremely difficult to ignite at atmospheric pressures even when the mixture contains as much as 10 weight percent of the combustion catalyst. Howseer, the insoluble Prussian blue containing ammonium nitrate mixture is readily ignitable when an elevated pressure such as 1,000 p.s.i. is imposed on the combustion chamber. This elevated pressure may be obtained quite readily by burning a small charge of black powder or cellulose nitrate in the combustion chamber. This black powder pressuring procedure is now commercially used in the so-called JATO units used for assisted take-off of airplanes. This insensitivity to ignition at atmospheric pressures and temperatures makes the insoluble Prussian blue ammonium nitrate mixture extremely safe in storage.

The gas generating composition of our invention comprises a mixture of ammonium nitrate and an effective amount of a combustion catalyst selected from the group consisting of ferric ferrocyanide and insoluble Prussian blue. In addition to the ammonium nitrate, moderate amounts of other inorganic nitrates, such as, potassium nitrate, sodium nitrate and magnesium nitrate may be present. We prefer to have present in the gas generating composition an oxidizable material, which material utilizes a part or all of the excess oxygen available from the decomposition of the ammonium nitrate (of course the catalyst uses some of the excess oxygen for the oxidation of the metal and carbon content thereof).

When operating with ammonium nitrate alone, large amounts of catalyst are needed to permit smooth burning of the composition; the amountof the catalyst, may be in excess of weight percentof the total composition.

When the gas generating composition consists essentially of ammonium nitrate, an oxidizable material and the combustion catalyst of this invention, the combustion catalyst may be present in an amount between about 0.5 and 25 weight percent, based on the total composition. In order to improve the ignitability of the composition and insure smooth burning thereof, it is preferred to use at least about 2% of catalyst. The burning rate is somewhat affected by the amount of catalyst present in the composition; however, the maximum effective concentration of the catalyst is about 10%. The amount of catalyst needed to obtain good ignitibility and smooth operation is dependent somewhat on the amount and type of oxidizable material present; in general between about 2 and 4 Weight percent will give good results when maximum burning rate is not a main consideration. When maximum burning rate is a main consideration, the amount of catalyst used should be between about 6 and 10 weight percent.

It has been discovered that ferric ferrocyanide per se is an effective combustion catalyst for the purposes of this invention. Also, it has been discovered that the form of ferric ferrocyanide known as insoluble Prussian blue is an eifective combustion catalyst for the purposes of this invention. While the chemical difference between ferric ferrocyanide and insoluble Prussian blue from various sources -is very slight, some difference does exist because ferric ferrocyanide and insoluble Prussian blue, from various sources, have different catalytic activity. It is intended to include within the scope of this invention ferric ferrocyanide and insoluble Prussian blue within the broad definition of these terms. The combustion catalysts of this invention include those materials which are hydrated or which are anhydrous.

The decomposition of ammonium nitrate produces free oxygen; additional energy can be obtained by the presence of an oxidizable material which combines with this free oxygen. Any material which contain a deficiency of oxygen can be utilized. Metals such as aluminum and magnesium may be added. The non-metallic elements sulfur and carbon can be used in our explosive mixture. Nitrogen-containing organic compounds that do not unduly sensitize the explosive mixture are particularly good; examples of these are urea, nitroguanidine, guanidine nitrate, and mononitrate naphthalene. Cellulosic materials are very desirable oxidizable materials, e.g., wood flour, cellulose acetate, etc. Because of their cheapness, hydrocarbon materials are a preferred oxidizable material. Examples of these are: parafiin waxes, petrolatum, high boiling hydrocarbon oils, tars, asphalts, bitumen, coal tar, shale oil residue, etc.

The amount of oxidizable material that may be added is dependent upon the particular type of material, but normally no more oxidizable material should be added than can react with the available oxygen to yield sootfree gas. When using hydrocarbon materials, in general, the maximum addition is between 20 and 25% by weight based on ammonium nitrate present. We prefer to use explosive mixtures which are about stoichiometrically balanced with respect to oxygen content.

The stability of ammonium nitrate in the presenceof our combustion catalyst was investigated by measuring the gas evolved from the mixture at various temperatures. A mixture of 94% ammonium nitrate and 6% ferric ferrocyanide was held for eight hours at 135 C.; no evolution of gas was detected. Fresh samples of the above mixtures were held at 170 C. for several hours; gas was given off to the extent of 3 to 4 ml. per hour per gram V 4 of sample. These tests show that a mixture of ammonium nitrate and our combustion catalyst is sufficiently stable at elevated temperature to permit formulation by a hot melt procedure.

The effectiveness of insoluble Prussian blue as a catalyst at elevated pressures is illustrated by the following examples. The grains of the'desired composition were made as follows: .When necessary, the ammonium nitrate was ground in a mortar to break up small lumps. The desired amount of ammonium nitrate, oxidizable material and combustion catalyst were weighed into a beaker and the contents thereof were thoroughly mixed. The mixture was extruded by means of a laboratory-size extruder to form a grain about /2 inch in diameter and 6 inches long. In order to insure uniformity, duplicate grains were made and tested in each example. The burning characteristics of each explosive mixture were determined after inhibiting the cylindrical. surface of the grain with a thin layer of either asphalt or a cellulose base thermoplastic such as cellulose acetate. The inhibited grain was placed in a bomb and the bomb brought to the desired operating pressure by burning a small amount of black powder therein. At this time the grain was ignited by means of a hot wire and the burning rate in inches per second was determined; each burning rate is an average of at least two trials.

Two test grains were made which differed only in the type of combustion catalyst used. Each test grain contained 74 weight percent of ammonium nitrate, 6% of cellulose acetate, 9% of ethylene glycol diglycolate and 9% dinitrodiphenyl oxide. The remainder, i.e., 2% of the grain, consisted of combustion catalyst. The two grains were burned in duplicateruns at a combustion chamber pressure of 1,000 psi. and the burning rate,

It was observed that these test grains burned to a fine black dust, much different from the fluffy ash given 01f by the conventional chromium combustion catalyst.

In order to test the effectiveness of a large grain, a miniature rocket motor was constructed. The combustion chamber of this motor was about 3 inches in diameter and about 9 inches long. At the gas exit end the chamber had a conical taper ending in a nozzle which had a diameter of 0.20 inch. In order to reduce the amount of composition used per test, an aluminum plug of approximately the diameter of the chamber and about 4 inches long Was inserted into the combustion chamber in some tests. The test grain normally was about 2.5 inches in diameter and about 4 inches long; the grain contained a coaxially located longitudinal hole 1 inch in diameter. The motor was operated by inserting the plug and the test grain. A charge of about 10 grams of a mixture of FF grade of black powder and sparkler powder was placed between the grain and the nozzle. An electrical squib was inserted into the chamber through the nozzle opening. The test was begun by firing the squib which fired the black powder; the burning of the black powder brought the chamber up to an operating pressure of about 1,000 psi. and ignited the test grain. It has been found that the amount of black powder ignitor will mally the test grain is burning smoothly in about 50 milliseconds.

The term ammonium nitrate as used in this specification and in the claims is intended to mean either ordinary commercial grade ammonium nitrate, such as, conventionally grained ammonium nitrate containing a small amount of impurities and which is then generally coated With a small amount of moisture-resisting material such as petrolatum or paraflin, or military grade ammonium nitrate, or a mixture of other inorganic nitrates and ammonium nitrate wherein the ammonium nitrate is the preponderate nitrate.

The explosive mixture can be made by milling the ingredients or by dry mixing; this operation is preferably followed by forming regular shaped compacted grains by pressing the powdered mix in molds. Itis preferred to prepare the shaped grains by adding the powdered ammonium nitrate to fused organic or oxidizable material at 100-125 C., mixing to form a paste, and pressing the paste into suitable molds. The cooled grains are strong and durable.

Having described the invention, what is claimed is:

l. A composition for the generation of gas which consists essentially of between about 5 and 25 weight percent of an oxidizable material, between about 2 and 10 References Cited in the file of this patent UNITED STATESPATENTS 43,021 Halvorson June 7, 1864 1,021,882 OBrien Apr. 2, 1912 1,071,949 OBrien Sept. 2, 1913 1,890,112 Fisher Dec. 6, 1932 2,159,234 Taylor May 23, 1939 FOREIGN PATENTS 14,196 Great Britain of 1897 

4. A COMPOSITION FOR THE GENERATION OF GAS WHICH CONSISTS ESSENTIALLY OF BETWEEN ABOUT 20 AND 25 WEIGHT PERCENT OF OXIDIZABLE HYDROCARBONS, BETWEEN ABOUT 2 AND 4 WEIGHT PERCENT OF INSOLUBLE PRUSSIAN BLUE COMBUSTION CATALYST AND THE REMAINDER AMMONIUM NITRATE. 